The present invention concerns a device for generating a first signal having a first frequency, including:
first generator means for generating a second signal having a second frequency that varies at least substantially parabolically as a function of the temperature with a first quadratic coefficient, which has a first maximum value at a first inversion temperature, and which has a first determined value at a reference temperature;
second generator means for generating a third signal having a third frequency which also varies at least substantially parabolically as a function of the temperature with a second quadratic coefficient, different from said first parabolic coefficient, which has a second maximum value at a second inversion temperature, and which has a second determined value at said reference temperature; and
mixing means for generating a fourth signal having a fourth frequency equal to the difference between said second and said third frequency.
Such a device is disclosed, for example, in Swiss Patent Nos. CH 626 500 and CH 631 315.
The two devices disclosed in these documents include a generator circuit which responds to the signal provided by the mixing circuit to generate correction pulses whose frequency depends upon that of the mixing signal, and thus upon the temperature. The output signal of these two devices is obtained by adding these correction pulses to the signal provided, after dividing its frequency, by one of the two oscillator circuits.
As a result of this arrangement, the frequency of the output signal provided by these devices is substantially independent of the temperature when it is measured over quite a long period. But also as a result of this arrangement, the frequency of the output signal exhibits abrupt variations at each appearance of a correction pulse. In other words, the frequency spectrum of this output signal has a very large number of lines of significant width, the position of these lines also varying with the temperature.
The devices disclosed in the aforementioned documents cannot therefore be used if it is necessary to have a signal with not only a temperature independent frequency but also a frequency spectrum having only a limited number of lines with fixed positions, which are also temperature independent. A signal having these properties is, for example, necessary when a high frequency signal picked up by an antenna, has to be synchronised, in a telecommunication device, with a low frequency signal generated in the device.
It is well known that oscillators including a so-called AT cut quartz resonator generate signals whose frequency is very stable as a function of the temperature. But, by nature, this frequency is quite high. If one wishes to make a device supplying a signal having a relatively low frequency from such an oscillator, it is thus necessary to associate a frequency divider circuit with the latter, which complicates the device and makes it more expensive. Moreover, the electric power consumed by such a frequency divider circuit is quite high because of the high frequency of the signal that it receives, which can be a serious drawback when the electric power has to be provided by a power source of small dimensions such as the battery of an electronic wristwatch.
One object of the present invention is thus to propose a device of the same type as those which are disclosed in the aforementioned patents but which does not have their drawbacks, which were also mentioned hereinbefore, i.e. a device generating an output signal having an at least substantially temperature independent frequency but also having a reduced number of lines, the position of these lines being also substantially temperature independent.
Another object of the present invention is to propose a device supplying a signal having a frequency, which has a variation as a function of the temperature, as low as that of the frequency of the signal provided by an oscillator including an AT cut quartz resonator, but which can be much lower than the latter.
The device according to the present invention whose features are listed in the annexed claim 1 achieves these objects.
As will be made clear hereinafter, as a result of these features the frequency of the signal supplied by a device according to the present invention is at least substantially temperature independent and does not exhibit any abrupt jump when the temperature varies. The frequency spectrum of this signal thus only has a small number of lines, and the position of these lines is also substantially temperature independent.
Moreover, as a result of these features the frequency of the signal provided by a device according to the present invention can be much lower than that of the signal provided by an oscillator including an AT cut quartz resonator. It is thus possible, in numerous cases, to use the signal provided by a device according to the present invention directly, without having to lower its frequency using a frequency divider circuit, which reduces the cost price and electric power consumption of the device. Furthermore, if a frequency divider circuit is, despite everything, associated with a device according to the present invention, its electric power consumption will be lower since the frequency of the signal provided by the device is low.